1. Field of the Invention
The present invention is generally drawn to brushless DC permanent magnet motors and more particularly to improved design disk type DC motors having permanent magnets on the rotor and stator coils.
2. Description of the Prior Art
A DC motor consists of two basic parts: a field-frame assembly or stator and an armature assembly or rotor. The armature rotates in a magnetic field that acts upon the current-carrying wires. The motor operates because a coil of wire that current is flowing through will rotate when placed in a magnetic field, until the coil aligns itself with the magnetic field. At the point just before the rotation would stop, brushes and a commutator are normally used to reverse the current in the coil. This commutator action causes the coil to continue rotating. The amount of torque developed in the coil depends on the strength of the magnetic field, the number of turns of wire in the coil, and the position of the coil in the magnetic field. Since there is a torque acting on each turn in the coil, the greater the number of turns, the greater the torque.
In most motors the magnetic field is furnished by an electromagnet, which can be made much stronger than a permanent magnet. The current to energize the electromagnet comes from the same source that supplies current to the armature.
Some applications for DC motors are: drives for food processing equipment, vacuum cleaners, main drives for electrical vehicles, drives of pumps, high speed centrifuges, gyros, etc.
Brushless DC motors have a very promising future as DC motors find more and more applications. The reason for the increased use of brushless DC motors comes from the fact that these motors have several significant advantages over old brush type motors. Brushless DC motors are more reliable due to the absence of the collector-brush set which is the most unreliable part of the DC motor. Also, brushless motors have better electromechanical and power output parameters.
Today, the use of brushless DC motors is found in a wide variety of devices for household electrical devices using only several watts of power to electrical vehicle motors using many kilowatts of power.
Nevertheless, in some areas where high power high RPM motors are required, the use of brushless DC motors is still limited. This is due to the fact that with the increase of RPM the loses due to heat generated in the iron stator of these motors increase as well. This leads to reduced motor efficiency and a reduction of the motor power output.
Obviously, avoiding the use of iron in brushless DC motor stators can thus seemingly solve the problem. However, in such cases the heat sink from the stator winding becomes very limited and motor overheating and destruction can easily occur. In such cases the need for a more efficient heat removal system for removing heat from the stator windings becomes imperative.
When brushless DC motors are designed, one of the main goals is to increase the reliability of the motor. The Rotor Position Sensor, it's design and location play a critical role in achieving this goal of motor increased reliability.
Efforts to solve the above described problems have been made by motor designers around the world as may be seen from numerous prior art patents which claim to have solved these mentioned problems.
PCT patent application WO84/01062 discloses a brushless disk-type DC motor which describes a rotor made as a ring shaped multipolar permanent magnet with the axial direction of the magnetic field being known. The permanent magnet rotor is located on a supporting plate and is magnetically connected by a ring magnetic circuit. The stator is made of flat windings which are electrically shifted with respect to each other by 90 degrees. The stator is mounted on the main motor frame and is axially aligned with the rotor permanent magnets. The stator winding rotating field (the stator does not have any iron) interacts with the permanent magnet polar field. Rotor Position Sensors are Hall devices mounted on the motor main frame and control alternate connections of the windings to the DC circuit.
The shortcoming of this described motor design is poor heat removal from the rotor windings because the motor fails to provide sufficient ventilation. This in turn leads to overheating of the coils, lower motor efficiency and reduced motor output power. These disadvantages become most pronounced for motors with medium and high power output (more than 100 W).
Another problem with this motor design comes from the use of one circular magnet as a rotor. This is not acceptable for high speed applications because the mechanical strength of permanent magnets is insufficient under these high RPM conditions.
Yet another shortcoming of this motor design comes from the mounting of the Hall devices on the stator itself. The magnetic field of the windings produces magnetic noise and interferes which interacts with the Hall devises to prevent their proper operation. All these forementioned design problems reduce the reliability of such prior art brushless DC motors.
Another known type of known brushless DC motor is disclosed in French Patent No. 70.01600. The motor disclosed therein has a multipolar rotor with a number of high coercive force bi-polar magnets arranged opposite to each other. Flat stator windings are located on a plate made of insulating material which plate is rigidly attached to the motor housing with the plane thereof oriented perpendicularly to the rotor axis. The active parts of the stator windings are oriented radially. The flat stator winding is located between the rotor parallel disks. This motor design with the use of separate permanent magnets attached to the disk instead of a single multipolar magnetic ring, as described in the previous design motor, allows the present motor to achieve higher speeds. However, even this design still faces the same problem of sufficient heat removal from the stator windings. Further, as may be best seen in the drawings and related description found in this patent, the ratio between the width of the magnet and the width of the active section winding located along the rotor disk radius is not optimal and thus does not provide maximum motor efficiency and maximum motor specific power.
Yet a third type of disk type brushless DC motor design in disclosed in Russian Patent SU1494877 A3. The motor described therein consists of a co-axially positioned stator and rotor. The rotor has permanent magnets. The stator has armature winding. The active surface of the winding faces the active surface of the permanent magnet poles with the same polarity. The armature windings are made as separate coils formed in a group located in the permanent magnet field with an angular displacement relative to each other. There is a switching device for an alternate connection of the group of coils of the armature to the DC bus. Also every coil or every permanent magnet is equipped with a device capable of regulating the force of magnetic interaction between the permanent magnet and the current in the coil in a circular direction. One of the motors is shown to have a stator with two active surfaces located opposite a mounted rotor disk. The second disk is a mirror image of the first one. Each disk is equipped with permanent magnets. The polarity of the rotor disk magnet located on one side of the stator is opposite to the polarity of the disk located on the other side of the stator.
This design improves mass and dimensional parameters but makes the use of magnets and the active parts of the stator winding less efficient. This lack of efficiency is due to the magnet blocking a part of the coil during its movement which blockage creates an opposite direction torque. Further, this design again fails to provide sufficient heat removal from the motor during high speed operation.
One of the main shortcomings of this last described design is that the Rotor Position Sensor (RPS) therein uses separate disks as signal elements. This complicates the motor design and leads to additional power losses. However, a benefit therefrom is a more stable operating unit due to the fact that the RPS is located outside of the stator winding field.
In view of the foregoing it will be seen that the prior art brushless DC motor designs failed to provide an simple construction efficient high speed brushless DC motor of optimized efficiently and reliability and having a cooling system for proper ventillation and heat removal during the whole operating range of motor speed or RPM.